Wireless communication systems having a base station and mobile station are susceptible to noise created by motion of the mobile station. Specifically, the motion causes (i) signal delay disturbances and (ii) changes in signal paths (i.e., reflections). In turn, the signal delay and changes in signal paths creates (i) noise and (ii) jitter in receiver timing. The noise and jitter cause errors in data bits of a communication signal between the base station and mobile station, which, in the worst case, manifests itself as a “dropped” connection, familiar to users of mobile phones. In less severe cases of voice communications, the errors in data bits results in static. In less severe cases of data communications, other effects are experienced, such as errors in data transfer between the two stations.
FIG. 1 is an illustration of a wireless communication system 100. The wireless communication system 100 includes a mobile station 105 having signaling paths 110 to an antenna 115. The antenna is coupled to a base station 120.
A signaling path 110 is a path along which radio frequency signals are transmitted between the mobile station 105 and antenna 115. The radio frequency signals, in the case of data communications, include data packets transmitting at typical modem baud rates, spanning between 9.6 kBaud and 57.6 kBaud. The maximum baud rate is typically possible where the signaling path 110 is conducive to low noise in the station 105 and/or base station 120. The slowest baud rate may be achievable in cases where the signaling path 110 retards communication between the mobile station 105 and/or base station 120.
In typical data communication applications, the mobile station 105 interacts with a remote web server on the Internet (not shown). The Internet is accessed through the base station 120 to an edge router (not shown). The edge router facilitates data packet transmission across the Internet to the remote web server having the website or files the user of the mobile station 105 wishes to access and/or download. The fastest baud rate, 57.6 kBaud, is desirable for downloading the web page or files from the remote web server.
Mobile computing is an application of data communications susceptible to motion induced noise. For example, a portable computer coupled to the Internet via wireless modem on a railway car experiences motion, but not always. When the railway car is at a railway station, there is no significant movement of the portable computer, so parameters affecting data communications, such as data transfer rate, can be optimized for the highest supported modem baud rate (e.g., 57.6 kBaud). However, when the railway car is moving, the data transfer rate is effectively reduced due to an increase in error rate caused by the motion of the personal computer.
It is well understood in the art that communication protocols, such as TCP/IP, which is a higher layer protocol, trap data transfer errors and request data retransmission of corrupted or noisy data. The amount of data requested for retransmission may be as much as an entire packet, which results in effective data transfer rate reduction analogous to re-reading sentences in a book, which reduces overall reading speed.
One approach to improving data transfer rates in the face of motion of the mobile station is to detect motion of the mobile station and, in turn, reduce the modem baud rate (e.g., 28.8 kBaud) between the base station and mobile station. By reducing the modem baud rate, the data transfer error rate is generally reduced. Although the reduced modem baud rate is less than optimal, the effective data transfer rate may be higher than operating at the faster modem baud rate in the presence of motion causing the high incidence of data re-transmissions.
A technique for determining motion of the mobile station is taught in U.S. Pat. No. 6,075,797 by Thomas, entitled “Method and System for Detecting Mobility of a Wireless-Capable Modem to Minimize Data Transfer Rate Renegotiations.” In Thomas, the mobile station having a wireless-capable modem detects motion of itself by analyzing the signal-to-noise ratio (SNR) as detected in received data at the wireless-capable modem. Examples of SNR analyses taught by Thomas include (i) statistical analysis of the SNR and (ii) discrete time sampling followed by frequency analysis of the SNR. If the mobile station detects motion, such as (i) by a comparison of a measured SNR sample to a specified SNR mean and variance and (ii) after a sufficient number of deviations from the mean and variance has occurred, the mobile station requests a modem baud rate reduction from the base station. Alternatively, if immobility or stationary status of the mobile station is returned, as determined again by an analysis of the SNR, then the mobile station requests a modem baud rate increase.